1. Field of the Invention
The invention relates to the use of certain porous polymeric membranes to toughen composite structures.
2. Description of Related Art
Toughness is a term used to refer to a material""s ability to adapt to, and ability to handle, stress. Tougher materials withstand greater stress prior to failure. Toughness often also implies failure that is more ductile in nature and non-catastrophic, characterized by inelastic and energy absorbing processes preceding failure. Frequently used metrics for toughness include but are not limited to fracture energy measurements, impact energy measurements, strength, puncture strength, impact strength, and strength after impact.
Related art in toughened composites includes rubber toughening of polymers, and composite reinforcement with fibrous material (chopped or continuous glass, aramid fibers, carbon fibers, etc.).
Rubber toughening is a term applied to materials toughened by the presence of a second and discrete rubber phase in a polymeric matrix. These rubber domains are believed to act as stress concentrators, acting to dissipate stress on a microscopic level and increase matrix toughness. Toughness is used herein to describe the ability to absorb energy in a non catastrophic manner. The rubber domains are typically generated by either a phase separation phenomenon or by addition of individual rubber particles. When added as individual rubber particles these particles are often in the form of core-shell rubbers where an outer shell is present over the rubber core. The shell is typically made from a material having a glass transition temperature above room temperature. The shell functions to prevent clumping and aid in dispersion. Rubber toughening can be accompanied by a drop in matrix glass transition temperature, a drop in resin modulus, or both simultaneously. Rubber toughening typically loses its effectiveness as the Tg of thermoset resins increases above 150xc2x0 C. as a result of the high cross-link density and inability of molecules to move without breakage in response to stress.
Thermoplastics that have been rubber toughened to enable applications beyond those available to the brittle neat polymer include polyvinyl chloride, polystyrene, acetal polymers including polyoxymethylene. Thermoset resins that have been rubber toughened include epoxy resins of low glass transition temperature.
Other polymers better known for their inherent toughness have also been enhanced by rubber toughening. Rubber toughened polycarbonate, and nylon, fall into this group.
Composite reinforcement in the traditional sense typically means increasing the overall stiffness of the matrix such that a composite can carry greater load with less deflection than the neat resin matrix. This reinforcement also often acts to increase the toughness of the material in that significantly more energy is required to generate a catastrophic failure than that of the neat resin matrix. Reinforcement is typically accomplished by addition of high modulus fibers such as glass, aramid, ceramic, or carbon fibers. These fibers typically fall in the 1-20 um diameter range. The coarseness of the fibrous reinforcement can limit applications, as for example in electronics where feature size can be on the order of one to several microns. To effectively boost stiffness and toughness in a polymeric composite the fibrous reinforcement is typically bonded to the resin to facilitate stress transfer from resin to fiber. This bond can be mechanical as in interlocking on a rough surface like that of carbon fiber, or it can be chemical as in covalent interaction between resin and fiber surface. This bonding is often enhanced with a reactive surface treatment to improve mechanical performance.
The prior art also discloses the use of polytetrafluoroethylene (PTFE) as composite reinforcement.
For example, it is known to prepare fibers from expanded PTFE, the fibers are used to produce fabrics that are then impregnated with thermosetting resins for use in printed circuit boards. These structures are not membrane structures and the fibers reinforce the resin on a macroscopic scale.
Also known is the use of fibrillated PTFE mixed with thermoplastic materials as well as thermosetting resins, wherein the fibrillated PTFE is discontinuous.
Composites have also been formed by the addition of fibrillated PTFE to a molybdenum disulfide and thermoplastic elastomer blend for improved abrasion resistance, solvent resistance and useful life and strength. Again, in such composites the PTFE is discontinuous.
The prior art also shows composites of fluorine containing elastomer and a fibrillated PTFE. The PTFE is discontinuous.
U.S. Pat. No. 3,953,566 to Gore discloses production of a form of PTFE known as expanded polytetrafluoroethylene (ePTFE), which is a porous membrane film of interconnected voids formed by nodes and fibrils. The void space in the ePTFE material comprises at least 50% of the volume, and frequently more than 70%. ePTFE is often a higher strength material than PTFE, and it is also an excellent dielectric material.
The use of such ePTFE porous membrane films to form composites is also known. For example, U.S. Pat. No. 5,753,358 to Korleski discloses an adhesive composite material comprising an ePTFE material having interconnected nodes and fibrils, wherein at least a portion of the void content of the material is imbibed with a particulate filled resin adhesive.
U.S. Pat. No. 4,784,901 to Hatakeyama et al. discloses flexible printed circuit board base materials comprising a sheet of porous, ePTFE impregnated with a bismaleimide-triazine resin. The sheet of porous, ePTFE comprises interconnected nodes and fibrils and voids.
U.S. Pat. No. 5,547,551 to Bahar et al. discloses ultra-thin composite membranes which include a base material and an ion exchange resin. The base material is a membrane which is defined by a thickness of less than 1 mil (0.025 mm) and a microstructure characterized by nodes and fibrils and voids, or in an alternative embodiment, by fibrils and voids with no nodes present. The ion exchange resin substantially impregnates the membrane such that the membrane is essentially air impermeable. Bahar discusses the improved performance of ion exchange membranes containing ePTFE over ion exchange resins without ePTFE. Important performance criteria discussed are uniformity and occlusiveness as in free of pin holes and air impermeability, mechanical integrity, and long term chemical stability. The membranes operate in the water swollen state where the ion exchange resin is highly swollen, soft and rubbery. Bahar indicates that a preferred base material is an expanded PTFE made in accordance with the teachings of U.S. Pat. No. 3,593,566.
U.S. Pat. No. 5,476,589 to Bacino discloses a non-woven web that is a thin, porous polytetrafluoroethylene membrane consisting essentially of a non-woven web having a microstructure of substantially only microfibrils fused at crossover points. The non-woven web is unusually strong, unusually thin, has unusually small pore sizes, but a very high air flow-through. It has a pore size between 0.05 and 0.4 micrometers; a bubble point between 10 and 60 psi; a pore size distribution value between 1.05 and 1.20; a ball burst strength between 0.9 and 17 pounds/force; and air flow of between 20 Frazier and 10 Gurley seconds; a thickness between 1.0 and 25.4 micrometers; and a fiber diameter ranging between 5 and 200 nm.
U.S. Pat. No. 5,288,547 to Elmes et al. discloses a process for preparing a composite using a porous membrane film component that enhances toughness in the obtained composite. Elmes et al. teach that the thermoplastic membrane dissolves into the composite and that it would be undesirable to not have the thermoplastic membrane dissolve into the composite. Elmes et al. also state that a weak resin-thermoplastic interface is a problem as it has a negative effect on composite performance.
The entire disclosure of each of the above U.S. Patents is hereby incorporated by reference.
Moreover, MICROLAM(copyright) 410 Dielectric and MICROLAM(copyright) 610 Dielectric are two commercially available products available from W.L. Gore and Associates, Newark, Del. These products are composites of thermoset resins and ePTFE porous membranes. MICROLAM(copyright) 410 Dielectric also contains a large volume fraction of inorganic filler and the membrane component generally has a maximum tensile modulus of 133 MPa and a tensile modulus of 1 MPa at 90 degrees from the direction of the maximum tensile modulus. FIG. 5 is an SEM of the type of membrane used in this product. MICROLAM(copyright) 610 Dielectric has a membrane component which generally has a maximum tensile modulus of 76 MPa and a tensile modulus of 16 MPa at 90 degrees from the direction of the maximum tensile modulus. FIG. 6 is an SEM of the type of membrane used in this product.
It is also known to produce materials including substantially node-free ePTFE membranes having the porosity at least partially imbibed with fluorinated ethylene propylene (xe2x80x9cFEPxe2x80x9d). FEP has a room temperature flexural modulus of about 0.5 to 0.7 GPa. Node free membranes imbibed with FEP typically exhibit a room temperature flexural modulus ratio of FEP/FEP-membrane composite of about 0.6.
The invention relates to the use of porous polymeric membrane films in composites such that these membranes provide substantially improved resistance to fracture and catastrophic failure in the composite. As used herein xe2x80x9ccompositexe2x80x9d means a body comprising two or more distinct materials. This toughening, in contrast to traditional rubber toughening is independent of the glass transition of the resin used. As used herein xe2x80x9cporous polymeric membrane filmxe2x80x9d means a porous polymeric film, the pores of which are substantially interconnected. The porous polymeric membrane film is insoluble in that it remains intact and undissolved during processing of the composite.
The porous polymeric membrane film satisfies the following equation:
75 MPa less than (longitudinal membrane tensile modulus+transverse membrane tensile modulus)/2.
In an aspect of the invention the composites include resin having a room temperature (23xc2x0 C.) flexural modulus of greater than about 1 GPa imbibed into at least a portion of the porosity of the membrane. The resin can be any suitable inorganic or organic material or a combination thereof which has a room temperature flexural modulus of greater than about 1 GPa. Suitable inorganic materials include, for example, metals, metalloids, and ceramics. Suitable organic materials include, for example, polymeric materials.
In a further aspect of the invention, the ratio of the room temperature flexural modulus of the resin to the room temperature flexural modulus of the composite, measured in the direction of the higher of the transverse and longitudinal moduli, is greater than or equal to about 1.
Toughening with such a membrane structure does not affect the glass S transition temperature (Tg) of the resin. Moreover, the Tg of the final composite is the same as the Tg of the neat resin without membrane. The impact on composite flexural and tensile modulus will depend upon the volume fraction of the membrane and the flexural and tensile moduli of the matrix and membrane. Because the membrane is a distinct and separate phase from the matrix, in contrast with rubber toughening, lowering of the flexural or tensile modulus by incomplete phase separation cannot occur.
It has been unexpectedly discovered that when used in a composite structure, porous polymeric membrane structures according to the invention contribute significantly to the fracture toughness of the composite. In an aspect of the invention the membrane structure is an expanded polytetrafluoroethylene membrane that has minimal material present in non-fibrillar form, termed xe2x80x9cnodesxe2x80x9d. In a further aspect of the invention the membrane is substantially void of nodal material. Isotropic fibril orientation is preferred when stress may be loaded from multiple directions. When stress is anisotropic it is preferred that the greater number of fibrils be parallel to the direction of maximum stress. When multiple layer structures are contemplated, cross plying of the layers may be desirable to maximize performance. One measure of fibril orientation and density is the membrane tensile modulus. Membranes having higher moduli are preferred.
Unlike traditional high modulus fiber reinforcements (e.g., glass, carbon, .etc.), the membranes of this invention have substantially non-linear, membrane-like structures. In the specific case of expanded polytetrafluoroethylene membranes the membrane does not readily wet or bond to other materials. Contrary to what the prior art teaches in the selection of a toughener or reinforcement material, the membranes of the invention unexpectedly provide enhanced composite performance.
Traditional reinforcements also provide for and act by substantially increasing the modulus in the composite over that of the neat resin alone. Carbon, graphite, glass, ceramic, and aramid fibers for example can increase the modulus of the composite by greater than a factor of 10.
In an aspect of the invention, composite room temperature flexural moduli measured in the direction of the higher of the membrane transverse and longitudinal moduli, are typically lower than the room temperature flexural modulus of the resin component alone.